Adhesive fixtures are used extensively in manufacturing industries to hold work-pieces relative to fabrication processes, assembly processes, and inspection processes. The types of work-pieces held include solid work-pieces, locked-out assembled work-pieces, and semi-locked out assembled work-pieces. The principle advantages to using this type of fixture in comparison to mechanical clamping fixtures, magnetic chucks, vacuum chucks, and electric-static chucks are: 1) enhanced access to the work-piece, 2) selectable holding strength and stiffness via choice of the adhesive, 3) diversity in the work-piece shapes, materials, and surface topographies that can be held, 4) ease of use and robustness of holding surfaces typically subject to significant geometric variation, and 5) the ability to induce less distortion into a geometrically complex, compliant work-piece prior to processing. These attributes are especially important to all processes that require the work-piece to be the held in its free state shape with sufficient stiffness.
FIGS. 1-4 illustrate typical adhesive work-holding devices. As matter of example, a system used for a machining process is provided. Regardless, the concepts presented are generalizable to all adhesive fixturing applications in the current state of the art.
FIG. 1 illustrates a fixture-work-piece assembly immediately after the adhesion process. The entire system is positioned at a prescribed location relative to a datum reference frame 1. The work-piece 2 is in contact with a system of fixture supports 3. The supports contact the work-piece at prescribed regions, thus registering its position relative to the datum reference frame. They also provide mechanical restraint.
Adhesive grippers 4 in combination with an adhesive 7 bond the work-piece relevant surfaces. In many applications, the adhesive grippers are permanently mounted to the fixture base (6). However in other applications they may not be. For many applications, the principal means to de-bond the work-piece is to move the gripper relative to the fixture base.
In addition, prior to the initiation of the adhesion process, it is possible that the grippers are not in the positions illustrated in FIG. 1. For example each could be moved to a position that provides clearance to the work-piece and enables it to be more easily loaded into the fixture. However at some point, each gripper is moved to the positions shown and its six d.d.o.f. (displacement degrees of freedom) are fully locked out relative to the fixture base. The d.d.o.f. of each gripper remains locked out indefinitely or until the time that the work-piece is de-bonded. By definition the adhesive grippers are part of the fixture base during this interval of time.
As a system, the supports and grippers completely lock out all possible d.d.o.f. between the work-piece surfaces, the fixture base, and the datum reference frame. Furthermore because the system is locked out, and because the adhesive fixture was designed correctly, the system has sufficient stiffness and holding strength to insure the success of the machining process. In general nearly every adhesive work-holding application requires a minimum level of holding stiffness and holding strength to be imparted by the adhesive fixture.
FIG. 2 illustrates another common practice. Here adhesive supports are used to both register the work-piece as well as provide adhered-mechanical restraint. Similar to the adhesive grippers, the d.d.o.f. of each adhesive support is fully locked out relative to the base prior to the adhesion process. Likewise it remains immobile till work-piece de-bonding or indefinitely.
FIG. 3 illustrates another common practice. Specifically external forces, represented by the arrows, are often applied to the work-piece to gently push it against the supports and adhesive supports prior to the activation of a structural adhesive in order to maintain it its registration. The external forces may be supplied by human hand, spring loaded clamps, pneumatically actuated clamps, hydraulically actuated clamps, magnetic force, or air pressure via a vacuum. These external forces are removed once the adhesive has hardened.
In the case of pressure sensitive adhesives, external forces are applied to the work-piece to push it against an adhesive support or adhesive gripper. This plastically deforms the tacky adhesive across the relevant surfaces and the fixing surfaces.
All of this activity creates elastic, preloaded contact stresses between the work-piece and the supports, adhesive supports, and adhesive. By design, these stresses are negligibly small and do not cause the work-piece to distort significantly.
FIG. 4 illustrates another common practice. In this case, the supports that are used to register the work-piece are removed in order to provide increased access to the work-piece. This is done with the understanding that the adhesive supports and adhesive grippers provide sufficient restraint to the work-piece and that the system has sufficient stiffness for the machining process.
It should be noted that that the assembly and bonding of the work-piece can be done anywhere. In some applications, the fixture base is permanently mounted to the machine in which the process is to be carried out. In other applications, the fixture base serves as a carrier that is mounted and dismounted to different machines used to carry out a sequence of processes. In this case, the fixture base typically incorporates a very precise kinematic coupling system that mates with a corresponding system in each machine. All of this is possible under the assumption that the datum reference frame, fixture base, and bonded work-piece traverse as a single rigid body.
Adhesive fixtures suffer two important limitations. The first limitation is that the adhesion process can exert stresses that distort the work-piece 2 from its free state shape 8 as shown in FIG. 5. If the distorted-displaced work-piece 2 is machined while held in this bonded state, the newly created surfaces may have negligible form error as illustrated in FIG. 6. Yet if the adhesive bonds are broken as illustrated in FIG. 7, the work-piece 2 will relax and take on a new free state shape 8. This will cause the previously machined surfaces to distort, and possibly go out of tolerance.
The means by which the work-piece distorts depends on what type of adhesive is used. In the case of a pressure sensitive adhesive, work-piece shape distortion is due to the flexure of the work-piece as it is being forced on to the adhesive. During this process the adhesive is plastically deforming as both the adhesive interface and the work-piece distortion continuously grow. Once the work-piece is secured and the external forces are removed, reactive elastic stresses at the adhesive interface will not let the work-piece relax back to its free state shape.
In general, work-piece distortion due to the adhesion process is exacerbated if the work-piece relevant surfaces are very compliant. It is also exacerbated by any variation in the external pressure applied to the work-piece as well as any local variation in adhesive thickness and compliance.
When structural adhesives are used, work-piece distortion results from two phenomena occurring at the same time. The first is the dynamic change in the elastic modulus and density of the adhesive as it solidifies. While the adhesive changes volume, it is adhered to both the relevant surface and the fixing surface. Normal stresses at the interfaces pull the surfaces together during shrinkage while they are pushed apart with expansion. The second is differential thermal expansion/contraction between the relevant surface, adhesive, and fixing surface. This is especially problematic for cases in which the three have significantly different coefficients of thermal expansion and when the adhesive must undergo a significant drop in temperature to solidify.
At the end of solidification, when the system has had the opportunity to uniformly reach, for example, 20° C., elastic stresses within the adhesive joints and at the interfaces prevent the work-piece from reaching its free state shape. It should be noted that in extreme cases when the compliance of a work-piece increases dramatically due to significant material removal, it is possible for the residual stresses in the adhesive joints to appreciably and dynamically distort the work-piece even further from the free state while it is still adhered in the fixture.
Work-piece distortion due to adhesive solidification increases with increased compliance of the relevant surface and an increase in residual stresses within the adhesive joint. Residual stresses generally increase with increased distance between the relevant surface and the fixing surface. They also generally increase with regard to the following adhesive properties: unrestrained, volumetric shrinkage percentage; modulus of elasticity; and coefficient of thermal expansion. They also increase with decreased compliance of the fixing surface.
The second limitation of an adhesive fixture is the restraint that it provides to a work-piece distorted by an imbalance of internal residual stresses. FIG. 8 illustrates a work-piece 2 that was theoretically adhered to a fixture without the development of residual, adhesive stresses and consequently was in its free state shape. This work-piece was subsequently machined, yielding a machined surface with negligible geometric error. Prior to machining, the internal stresses within the work-piece were in equilibrium. Unfortunately the machining process removes a portion of these internal stresses, causing an imbalance, which also results in stresses arising in the adhesive joints. To relieve the internal strain energy arising from this imbalance, the work-piece must obtain a new free state shape. However the restraint provided by the adhesive joints prevents this. Consequently the machined surface is free from form error while adhered in the fixture.
FIG. 9 illustrates the work-piece 2 obtaining its free state shape 8 once the adhesive joints are removed. In this case, the machined surface is now distorted. This phenomenon occurs to varying degree for all material removal processes and material addition processes.
The big problem with this phenomena is that the geometry of a part must often be characterized while it is in the free state. Consequently the adhesive bonds must be broken prior to inspection. If the part is out of tolerance, it will have to be re-bonded to the fixture and re-processed.
In many cases it is very difficult to eliminate work-piece shape distortion that results from the combination of adhesion stresses and imbalanced internal stresses. It is not unusual for the work-piece to undergo many cycles in which it is bonded to the fixture, processed, de-bonded, and measured to verify specifications or to determine necessary correction in the manufacturing process. All of this leads to increased manufacturing lead time and increased manufacturing cost.
In general, this problem is more significant if adhesive joints restrain surfaces that are highly compliant at any stage of the manufacturing process. Unfortunately, compliant work-piece surfaces are the ones that are in the greatest need of support from an adhesive joint in order to counteract external stresses supplied by the manufacturing process. Yet forming adhesive joints on these surfaces can cause significantly greater distortion than the manufacturing process.
Both problems just described could be eliminated entirely if the elastic stresses within the adhesive joints could be minimized on-demand, whenever the work-piece is free from external loading. Theoretically this could happen if the adhesive can strain via creep. The creep modulus of many adhesives increases with increased temperature.
While not related to adhesive work-holding, this is a common technique employed in welding processes. To relieve highly stressed welded joints, the entire welded assembly is placed into an oven and allowed to thermally soak at elevated temperatures. This releases stresses within the joints and allows the structure to obtain a new equilibrium shape.
While this process is suitable for welding, it is not particularly attractive for accurate work-holding. Not only is it time consuming and cumbersome, but there is no guarantee it will lead to the desired result. At the elevated temperature, it may be possible to achieve stress free joints, but during cool down, heat transfer and differing thermal contraction may cause residual stresses to re-appear.
There are other approaches advocated in the prior art. All of them address the reduction of work-piece distortion due to the adhesion process. With regard to the use of pressure sensitive adhesives for work-holding of thin, flat, ultra-compliant silicon wafers, U.S. Pat. No. 6,398,892 advocates the use of an extremely flat and rigid adhesive support to minimize localized stiffness and thickness variation.
U.S. Pat. No. 5,624,521 explains that one of the best ways to reduce shrinkage distortion during lens blocking is to minimize the relevant surface area in contact with adhesive. Furthermore it is far better to use a distribution of a large number of small, disconnected adhesive patches rather than one large contiguous patch.
Another approach is to select an adhesive chemistry that provides the desired mechanical properties, but induces minimal shrinkage stresses. For example, a common approach for polymer adhesives is to choose one with either a very low elastic modulus or one with a very low volumetric shrinkage percentage. A popular way to accomplish the latter is to add inert filler (glass, ceramics, pre-polymerized adhesive, etc.) into the uncured adhesive to thus reduce the total volume of adhesive that needs to be polymerized. This approach is advocated by US Patent No. US Pub US2010/0170635 for lens blocking. It is also highly desired to choose a polymer adhesive with a glass transition temperature well above the highest temperature expected in the manufacturing process in order to minimize thermal growth of the adhesive.
With regard to the use of thermo-plastics and low temperature melt alloys, it is typically desirable to choose one with some combination of low volumetric shrinkage percentage, low coefficient of thermal expansion, and low solidus temperature. The latter is typically chosen to not to be too far above the maximum operating temperature of the manufacturing process.
Other approaches can be adapted from the general area of adhesive assembly. Some important considerations are:                1. The displacement of an adhered component can be minimized if the external stresses acting on its relevant surfaces cancel each other. In this case, the work-piece will distort, but not displace. This can be controlled through joint design and the control of thickness variation in the adhesive joints. (see U.S. Pat. Nos. 6,000,784, 6,596,104)        2. The use of thinner adhesive joints and less adhesive can minimize assembly distortion and misalignment. (see U.S. Pat. No. 6,000,784)        3. Curing adhesive patches in sequence rather than simultaneously can significantly reduce both work-piece distortion and work-piece displacement. If adhesive joints are cured in sequence, solidified joints can stiffen relevant surfaces yet to be bonded. (see U.S. Pat. No. 6,000,784).        4. Adhesively bonding intermediary components into an assembly can reduce the alignment error of critical components. Basically the addition of intermediary components gives more design freedom for joint design and curing sequence. (see U.S. Pat. No. 6,000,784)        5. When bonding a relevant surface, distortion and displacement can be reduced if a low volume shrinkage percentage adhesive is used in combination with a principal adhesive that induces higher shrinkage stress. By first solidifying the low shrink adhesive, it is possible to stiffen the relevant surface. This significantly reduces distortion when the principal adhesive is solidified. (see U.S. Pat. No. 6,302,512)        6. If adhesive joints are solidified simultaneously, work-piece distortion and displacement can be minimized by controlling the solidification rate and heat transfer at different joints or at different regions within an adhesive joint. (see U.S. Pat. No. 7,918,963 B2)All of the techniques listed here can be applied with varying degrees of success to minimize work-piece distortion and work-piece displacement during the adhesion process. But none of these techniques address the second problem.        
One way to minimize the second problem is to reduce residual internal stresses within the work-piece prior to bonding. With this in mind, it is not uncommon for metallic work-pieces to be stress relieved by thermal heat treatment prior to material removal processes, regardless of holding method. In these cases, the work-pieces are allowed to cool to room temperature before mounting. This is an effective technique, but it has a number of limitations. It cannot be universally applied to all work-piece materials. It sometimes leads to unwanted shape distortion of pre-existing features. It is an added step that costs time and money. Furthermore it cannot typically be applied while the part is held adhesively. Most value added materials have stress relief temperatures which will destroy many practical work-holding adhesives. They can also damage the adhesive fixture.